The present invention relates to radio frequency antennas and more particularly, to loop antennas which generate fields that are generally canceling at distances of one wavelength or more from the antenna.
In certain known types of electronic systems it is known to provide one or more loop antennas wherein coupling between an antenna and its proximate surrounding is high, but wherein the design of the antenna is such that coupling between the antenna and its distant surrounding (i.e., about one wavelength or more distant from the antenna) is minimized. Such antennas are generally used for near-field communications or sensing applications where the term "near field" means within one half wavelength of the antenna. Examples of such applications include communications with implanted medical devices, short range wireless local area communications networks for computers and radio frequency identification systems including electronic article surveillance (EAS) systems. Generally, the coupling to these loop antennas is primarily via magnetic induction.
For example, radio frequency EAS systems usually include both a transmit antenna and a receive antenna which collectively establish a surveillance zone, and tags which are attached to articles being protected. The transmit antenna generates a variable frequency electromagnetic field within a range of a first predetermined frequency. The tags each include a resonant circuit having a predetermined resonant frequency generally equal to the first frequency. When one of the tags is present in the surveillance zone, the field generated by the transmit antenna induces a voltage in the resonant circuit in the tag, which causes the resonant circuit to generate an electromagnetic field, causing a disturbance in the field within the surveillance zone. The receive antenna detects the electromagnetic field disturbance and generates a signal indicating the presence of the tag (and thus, the protected article attached to the tag) in the surveillance zone.
The design of these antennas should satisfy two objectives: (1) to maximize the coupling to the tag over as wide a distance between the transmit and receive antennas as possible, and (2) to minimize the coupling to the far-field. These are conflicting objectives. Prior art antennas, such as those described by Lichtblau in U.S. Pat. Nos. 4,243,980, 4,260,990 and 4,866,455, herein incorporated by reference, generally incorporate two or more loops such that in combination the sizes of each loop, the magnitude of the currents within the loops and the direction of the currents generate fields which, when measured at a point distant from the antenna, generally cancel. In other words, the fields created from each of the loops, when summed, net a field which approaches zero. Such far-field cancellation is not possible when only one loop is used. In figure-eight loop antennas, the loops are generally rectangular, arranged in a coplanar configuration, and offset in position such that at least one side of each loop is proximate to a side of another loop. In other words, the shared sides are immediately adjacent to each other. Lichtblau further discloses in U.S. Pat. Nos. 4,251,808 and 4,866,455, herein incorporated by reference, antennas with shields that are used to prevent electric field coupling to the antennas, but does not disclose any improvement relating to satisfying the two above-stated objectives.
Bowers discloses in U.S. patent application Ser. No. 08/482,680 filed Jun. 7, 1995, now U.S. Pat. No. 5,602,556 an improved two loop (figure 8) configuration as an optional element of a composite antenna, the properties of which include both good far-field cancellation and the generation of rotating fields. The improvement in the two-loop configuration comprises separating the loops from each other such that the shared sides are no longer shared or immediately adjacent to each other. This improvement causes the diameter of the toroid-shaped zone of high coupling proximate to the antenna to be increased, thereby increasing the distance by which the transmit and receive antennas of an EAS system may be separated. However, there is no improvement in this antenna as it relates to the second-stated objective of minimizing coupling to the far-field.
The present invention provides an antenna having both much reduced far-field coupling properties and increased coupling in a zone proximate to the antenna. Generally, the antenna comprises first and second triangular loops of generally equal dimensions and shape wherein the loops are coplanar and positioned on opposite sides of a central axis in the plane of the loops. In addition, the loops are positioned such that one corner of the loops, an outside corner, is proximate to or intersects a corner of a coplanar rectangle defining the outside dimensions of the antenna. The loops are connected to each other by a crossover with a length at least equal to a length of the shortest side of the loops such that when connected to a drive circuit, the current in the loops flows in opposite directions and thereby generates substantially canceling fields. A preferred embodiment of the invention comprises inverting, flipping or mirroring the orientation of the second loop relative to the first loop such that outside corners of the loops are in diagonally opposite corners of the dimension defining rectangle. The antenna can be connected to a transmitting or drive circuit which provides relatively high current and still meet regulatory requirements for far-field radiation. The present invention also provides an antenna which is highly sensitive to externally emitted signals within a zone proximate to the antenna, but highly insensitive to distant emitted signals.